This is part of the Anatomically Correct Series and has observed all updated rules for that series.
Since I have already created a Hell designed by creatures who are inspired by the Lovecraftian Mi-Go race, I now need to try to come up with an anatomically correct alien species that fits the description, in more modern terms. Note: These creatures occupy the world I designed. In no possible way could an explanation for my creature derive from the C'thulu mythos or any Lovecraftian worlds. Reference to commercial products are only for descriptive purposes, and no commercial third-party product is being amended or used in any possible way.
- magic: I personally have never once used magic in my writing, and don't like to do so. Realistically, this alien shares the characteristics of a god-like creature in a world chock full of gods and things which blatantly defy physics. Lovecraft, a fervid student of physics, employed his understanding of natural law to effect his horror by finding holes and shaking our faith in them. So I accept that I may not be able to escape using some supernatural forces for this creature. My goal is to make it as minimal as possible: thus, if a physical answer can explain it, then a magical one shall not. The science-based tag trumps the magic tag everywhere possible. Also, the "magic," if needed, won't be intelligent or "god-derived;" it will be a simple alteration of some force or physical property—preferably quantum probabilities of light/particles/matter becoming non-Gausian—inexplicably connected with their corporeal form.
Single cause answer
My goal in an answer is to have a single feature of this creature be the cause of the three listed side effects. E.g., should we want a world that has all features of liquid water, precipitation, and navigable waterways, this could be built from three separate questions with a potpourri of incompatible solutions, or we could simply say the world is in the "goldilocks zone" and the world has better believability.
Research: The description of the Mi-Go varies from different reports. My world has no preference in this physical appearance as it may not even be the creature’s true form (see side effect 2). The only commonly accepted fact about my creature is that they are essentially fungi in their biology. A couple drawn pictures are included below from different reports.
And another artist rendition of a Mi-Go with a brain in a vat:
1. Physical side effect: Locomotion through the environment
The Mi-Go-like creatures originate in space, using energy in an extremely exotic way, that contributes to the three side-effects. Research: On earth, we have discovered bacteria deep in the crust which actually feeds on radioactive decay. I considered that a highly radioactive protoplanet or other body could serve as a likely birthplace for these creatures in an answer. They can also survive in and travel through "the aether" (space) by some method of locomotion. For this, my research suggests there may be either some sort of reaction engine such as ion propulsion, or if this is infeasible, a supernatural manipulation of forces will need to explain this. (Note that the Mi-Go did walk around and leave "claw tracks," but my creatures do not require this. So they do not need to be ambulatory or very massive since that will complicate flight and space travel.)
2. Optical illusion side effect: Inability to record
The Mi-Go can not be recorded on camera film or other capture media. Neither can mine due to this characteristic of the world. It does not ever say that they are "invisible" on these media, but neither their form nor image will reproduce accurately on a film or other media. Like the Mi-Go, they can apparently be described by a human observer, however, so unless a more scientifically compatible answer exists, I accept that human cones and rods can detect and signal light emissions from them. Research: The visions which reach a human mind are not simple reproductions of what reaches our eyes. A rare and horrifying disorder causes you to see the right half of human faces as if they are melting, which leads scientists to conclude that our brains are doing a significant amount of image processing before we recognize what we see. In an answer, "hallucinogenic" is a scientifically sound explanation for the ability for a human to describe this creature: Our description of them does not in fact come from what reaches our eyes, but instead, they actively or passively "cause" the form of their appearance in our minds. The world can sidestep a complicated explanation with light working differently on film and on a retina. The lore on Mi-Go tells that they are composed of an unnatural form of matter which prevents light from rendering an accurate image of them. My world rejects exotic forms of matter which are not scientifically sound within our more modern concepts. In an answer, this optical obscurity, whether it be hallucinogenic or otherwise, is again preferred to share the same cause as the creature's ability to fly and traverse space; "magic" being a last resort fall-back.
The Mi-Go appear to fly in fluids by means of wings, which flap slowly. In an answer, I can accept that this may also be part of their hallucination effect if actual "wings" don't contribute to the answer.
3. Psychosocial side effect: Brains
They have a need or use for human servants as brains in a vat. They need our intelligence for some purpose, but our bodies are incompatible (or unnecessary) for that purpose. So they only take our brains and keep them alive. Whatever odd feature of the world that causes the other traits in this question should also directly relate to this need for live human intellect. Obviously not as a "physical" need, but because that feature of this world which causes their mobility and optical obscurity has left them needing human intellect, or perhaps some portion of our processing ability to interface with "the real world." Research: Mi-Go are extremely intelligent but have vastly incompatible moral values to us, which my creatures share. So the need for our intellect may relate to that. Clarification: The jar is explained simply because transporting the mass of a whole human body is too cumbersome. They don't need a human, they need human intellect. So they discard the 98% of a human that they do not need.